Correlation between lung and plasma angiotensin converting enzyme and the hypotensive effect of captopril in conscious rabbits.

We measured both pulmonary and plasma angiotensin converting enzyme (ACE) activity in conscious rabbits before and for 6 days after administration of captopril (2 mg/kg i.v.). Pulmonary ACE activity was measured by means of modified indicator-dilution techniques after bolus injection of [3H]benzoyl-phenyl-alanyl-alanyl-proline ( BPAP , synthetic substrate for ACE). Plasma ACE activity was also determined radiometrically with [3H] BPAP as substrate. In addition, we measured the systemic pressor response to i.v. bolus dose of angiotensin I both before and after captopril. Twenty-four hours after captopril, pulmonary metabolism of BPAP had decreased from control of 73 +/- 5 to 8 +/- 2%; even 6 days after drug treatment there was still evidence of inhibition. In contrast, plasma ACE activity was significantly (P less than .05) reduced within 15 min of treatment (to 20% of control levels) but recovered within 24 hr. The time course of changes in the pressor response to angiotensin I, after captopril, resembled that of plasma ACE activity. Mean systemic arterial blood pressure was 81 +/- 4 torr at control and reached its nadir at 24 hr (64 +/- 2 torr; P less than .05); thereafter a gradual recovery ensued, similar in time course to that of pulmonary ACE. These data suggest that inhibition of plasma ACE and also the pressor response to angiotensin I are unrelated temporally to the hypotensive effects of captopril.